A stacked package, in which a plurality of semiconductor chips are stacked, is one solution for the demand of smaller, lighter, and higher functioning electric devices including portable devices.
In assembling the stacked package, as the wire bonding method for making an electrical connection between the semiconductor chips and the substrate, a ball bonding (nail-head bonding) method using a gold wire, or a wedge bonding method using an aluminium wire is adopted.
In the wedge bonding method, in order to form a loop in a radial pattern, it is required to rotate a bonding head or a substrate because of directivity in wire directions. Also, in the wedge bonding method, when the semiconductor chip of the upper layer is small, it is required to position a bonding pad on the substrate away from an end of the semiconductor chip of the bottom layer because it is difficult to flex the wire. As a result, the package size is increased. Note that, the wedge bonding method commonly adopts the forward method in which the wire is first bonded on a bonding pad on the semiconductor chip and then second bonded on the bonding pad on the substrate.
Thus, to reduce the package size, the reverse method has been used, in which the order of connecting the wire is reversed, i.e., the first bonding is carried out on the bonding pad on the substrate and then the second bonding on the bonding pad of the semiconductor chip of the upper layer of stacked layers (Japanese Unexamined Patent Publication No. 116849/1998 (Tokukaihei 10-116849) (published date May 6, 1998)). The reverse method allows the wire to be shaped into the form of a “shoulder”, by which the bonding pad on the substrate can be positioned closer to the end of the semiconductor chip of the bottom layer, thus reducing the package size.
However, while the reverse method of the wedge bonding method is effective when the semiconductor chips of the upper and lower layers have substantially the same size, when the semiconductor chip of the upper layer is smaller than the semiconductor chip of the lower layer, the wire length is increased. Further, because the aluminium wire used in wedge bonding is crushed to make the connection, if the wire is jiggled up and down after first bonding to change the shape of the wire loop, the connection becomes weak. Thus, in the reverse method of the wedge bonding method, it is difficult to bend the wire at an angle near the right angle, and the wire takes a circular shape with a round shoulder, thus making it difficult to reduce the size of the device.
On the other hand, in the ball bonding method, there is no directivity in wire directions, and thus it is not required to rotate the substrate, etc. and the loop can be made quickly, and therefore this method is suitable for mass production. Further, because the wire directions can be freely set, it offers a large degree of freedom in positioning of the bonding pad on the substrate. Moreover, second bonding can be made on the same bonding pad.
Namely, as shown in FIG. 9(a), with the ball bonding method, the wire can be flexed easily, and the bonding pad on the substrate can be positioned closer to the end of the semiconductor chip of the bottom layer, and thus the ball bonding method is suitable for miniaturization of the device.
Here, the ball bonding method for bonding the semiconductor chip with the substrate commonly adopts the forward method in which the wire is first bonded on the bonding pad on the semiconductor chip and then second bonded on the bonding pad on the substrate.
However, as shown in FIG. 9(a), in the bonding adopting the forward method, the flat length La is usually only about half the wire length Lb. Thus, when the forward method is used for the bonding of semiconductor chips 2 and 3 on the upper side of the semiconductor chip 1 with the substrate 4 to maintain a clearance from the wire of the lower layer, it is required to provide a sufficient distance from the second bonding position of the lower layer. As a result, the distance Lc from the end of the semiconductor chip 1 of the bottom layer to the bonding pad on the substrate 4 is increased, which in turn increases the package size.
For this problem, as shown in FIG. 9(b), by bringing the ends of the semiconductor chips 2 and 3 closer to the end of the semiconductor chip 1 of the lower layer, the flat length La can be reduced, which in turn reduces the wire length Lb and the distance Lc, thus reducing the package size. However, on the opposite side of the substrate 4, the bonding pad is moved away and the distance Lc is increased in return on the opposite side. Also, when the semiconductor chips 2 and 3 of the upper layers are increased in size to reduce the distance Lc, the yield of the chips is reduced, making the method unsuitable.
Further, in first bonding, in order to flex the wire in the form of a shoulder, it is required to move a capillary in a direction away from the bonding pad on the substrate. In this instance, there is a possibility that the capillary comes into contact with the end of the semiconductor chip of the upper layer (FIG. 10(a)), or the wire contacts the end of the semiconductor chip of the upper layer (FIG. 10(b)), and thus it is required to provide a sufficient distance between the bonding pad of the semiconductor chip of the lower layer and the end of the semiconductor chip of the upper layer.
As described, the conventional wire bonding method adopting the forward method has the problem that many restrictions are imposed on combinations of semiconductor chips to be stacked in wire bonding of the multi-layered semiconductor device.